The myelin sheath is essential to normal conduction in nerves and is altered in multiple sclerosis and Guillain-Barre diseases. Understanding how myelin is formed and repaired requires basic studies of the differentiation of myelin-forming cells both in vitro and in vivo. We are now studying the precursor cells of oligodendrocytes in the developing rat CNS and the molecules that trigger their mitosis and differentiation. We found that the major mitogen for the 0-2A progenitor cells is a PDGF-like molecule synthesized by Type 1 astrocytes and that TGF beta, also synthesized by these cells, may modulate the PDGF effects. After a number of mitoses driven by PDGF, the 0-2A progenitor evolves into another precursor which acquires 04 antigen, loses vimentin, divides more slowly and can be triggered to become an oligodendrocyte by IGF-1 or insulin. We have also shown that 0- 2A progenitors sorted from the brain have properties identical to those characterized in optic nerve, but have higher mitogenic potential. Investigations are being carried out to determine if CNS remyelination recapitulates cellular and molecular events of myelination by using immunocytochemistry on frozen sections combined with tritiated thymidine pulse. Characterization of the dividing precursors of oligodendrocytes around the lesions of animals with active demyelination produced by a corona virus are also being performed. Using in situ hybridization and probes specific for some myelin gene exons, we are examining whether splicing events in remyelination occur as in myelination. Additional studies on myelin deficient rats are aimed at analyzing how differentiation of oligodendrocytes is regulated when expression of one major myelin gene, proteolipid, is altered. Expression of proteolipid was unexpectedly found to occur in Schwann cells although peripheral myelin does not contain this protein.